Rarity = people willing to spend more to have it. Case in point: diamonds vs. glass. They look the same, but people are willing to spend ridiculous amounts of money just to know they have something "special".
(Yes, I know diamond is super-hard and thus useful in ways glass cannot be. But that demand is far lower than the demand for jewellery, and if it were only needed for that then diamonds would be way, way cheaper.)
(Yes, I know diamond is super-hard and thus useful in ways glass cannot be. But that demand is far lower than the demand for jewellery, and if it were only needed for that then diamonds would be way, way cheaper.)